Fix XRayTest link on FreeBSD (and likely NetBSD too)

Summary:
As reported on llvm-testers, during 8.0.0-rc1 testing I got errors while
building of `XRayTest`, during `check-all`:

```
[100%] Generating XRayTest-x86_64-Test
/home/dim/llvm/8.0.0/rc1/Phase3/Release/llvmCore-8.0.0-rc1.obj/./lib/libLLVMSupport.a(Signals.cpp.o): In function `llvm::sys::PrintStackTrace(llvm::raw_ostream&)':
Signals.cpp:(.text._ZN4llvm3sys15PrintStackTraceERNS_11raw_ostreamE+0x24): undefined reference to `backtrace'
Signals.cpp:(.text._ZN4llvm3sys15PrintStackTraceERNS_11raw_ostreamE+0x254): undefined reference to `llvm::itaniumDemangle(char const*, char*, unsigned long*, int*)'
clang-8: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[3]: *** [projects/compiler-rt/lib/xray/tests/unit/CMakeFiles/TXRayTest-x86_64-Test.dir/build.make:73: projects/compiler-rt/lib/xray/tests/unit/XRayTest-x86_64-Test] Error 1
gmake[3]: Target 'projects/compiler-rt/lib/xray/tests/unit/CMakeFiles/TXRayTest-x86_64-Test.dir/build' not remade because of errors.
gmake[2]: *** [CMakeFiles/Makefile2:33513: projects/compiler-rt/lib/xray/tests/unit/CMakeFiles/TXRayTest-x86_64-Test.dir/all] Error 2
gmake[2]: Target 'CMakeFiles/check-all.dir/all' not remade because of errors.
gmake[1]: *** [CMakeFiles/Makefile2:737: CMakeFiles/check-all.dir/rule] Error 2
gmake[1]: Target 'check-all' not remade because of errors.
gmake: *** [Makefile:277: check-all] Error 2
[Release Phase3] check-all failed
```

This is because the `backtrace` function requires `-lexecinfo` on BSD
platforms.  To fix this, detect the `execinfo` library in
`cmake/config-ix.cmake`, and add it to the unit test link flags.

Additionally, since the code in `sys::PrintStackTrace` makes use of
`itaniumDemangle`, also add `-lLLVMDemangle`.  (Note that this is more
of a general problem with libLLVMSupport, but I'm looking for a quick
fix now so it can be merged to the 8.0 branch.)

Reviewers: dberris, hans, mgorny, samsonov

Reviewed By: dberris

Subscribers: krytarowski, delcypher, erik.pilkington, #sanitizers, emaste, llvm-commits

Differential Revision: https://reviews.llvm.org/D57181

llvm-svn: 352234